This invention is in part an outgrowth of the subject-matter of U.S. Pat. No. 3,735,317; May 22, 1973 to Philip C. Jacobs, Jr. for ELECTRIC MULTIBREAK FORMING CARTRIDGE FUSE and is in another part an outgrowth of the subject-matter of U.S. Pat. No. 3,848,214 to Erwin Salzer, Nov. 12, 1974 for METHOD OF ASSEMBLING ELECTRIC HIGH-VOLTAGE FUSES AND SUBASSEMBLY THEREFOR.
The above patent to Jacobs solves the problem of forming series breaks in a fusible element incident to occurrence of small protracted overloads. The fusible element or fuse link is provided with a link-severing low fusing point overlay which forms a break at very small overloads, e.g., the 1 hour fusing current. The fuse of Jacobs includes a shunt across the overlay portion of the fuse link which shunt is adapted to generate relatively large amounts of heat at the ends thereof conductively connected to the fusible element at opposite sides of said overlay thereof. Normally the shunt does not carry any significant current on account of its high resistance. The shunt begins to carry a significant current following formation of a break by a metallurgical reaction between the base metal of the fusible element and the overlay metal at the point of said overlay. Thereafter the ends of the shunt that generate relatively large amounts of heat fuse and kindle arcs between the fusible element and its shunt. These arcs are supposed to form two additional breaks in the fusible element, one to each side of the overlay thereof. Because these arcs are relatively low current arcs, the time involved in forming the two aforementioned additional breaks in the fusible element is relatively long.
It is, therefore, one object of this invention to provide means that contaminate the arc gaps formed between the fusible element and its shunt by evolution of highly ionizable metal vapors, thus greatly accelerating the formation of breaks in the fusible element to both sides of said overlay thereof.
Another object of this invention is to provide simple means for conductively connecting the ends of said shunt to points of said fusible element situated to opposite sides of said overlay thereof.
The aforementioned patent to Salzer discloses a fixture for making fuses for elevated circuit voltages. The cylindrical plug terminals of the fuse form a part of the fixture and are tied together by a plurality of metal rods extending parallel to the common axis of the plug terminals. Thus the plug terminals and said plurality of metal rods form jointly a squirrel-cage-like structure. A fusible element is wound helically around the metal rods and its ends are conductively connected to the axially inner end surfaces of the plug terminals. Thereafter the entire assembly is inserted in coaxial relation into a tubular casing of electric insulating material and the ends of the latter are pinned to the plug terminals. Then the casing is filled with a pulverulent arc-quenching filler, preferably quartz sand. After the pulverulent arc-quenching filler has been duly compacted it is capable of supporting the helically wound fusible element or elements. Thus the fusible element supporting rods are not needed any longer and may be withdrawn in a direction longitudinally thereof from the fusible element and removed from the casing by appropriate bores in one of the plug terminals. The resulting high-voltage fuses contain a relatively higher volume of pulverulent arc-quenching filler than conventional high-voltage fuses, i.e. the filler volume is increased by a volume equal to that normally occupied by mandrel-like solid supports for the fusible element. Elimination of such supports is further desirable because of their relatively high cost, the danger of becoming more or less conductive under the heat of high-current arcs, the danger of evolving too much gas under the action of such arcs, etc. Fuses as shown in the above patent to Salzer do not lend themselves to interruption of protracted very small overload currents, e.g. the three hour fusing current.
It is, therefore, another object of this invention to modify the fuses disclosed in the above patent to Salzer in such a way as to impart them with the ability of effectively interrupting small protracted overload currents. A more specific object of this invention is to modify the type of fuses disclosed in the above patent to Salzer in such a way as to adapt the same to the low overload current-interrupting means disclosed in the above Jacobs patent.
A still narrower object of this invention is to modify the fuses disclosed in the above patent to Salzer by incorporation into the same of a greatly improved version of the protracted low overload current series multibreak forming means of Jacobs.